This invention relates to a fluid flow control valve responsive to both temperature and pressure conditions, and particularly a fluid flow control valve for regulating engine cooling fan operation.
One type of known engine cooling fan drive is fluid operated, e.g. an oil torus fan drive, wherein fan speed is controllably decreased or increased by the amount of fluid allowed to flow out of the drive or retained in the drive, to vary the amount of cooling air propelled through the radiator containing circulating engine coolant liquid. It has been known to provide a temperature responsive control valve that decreases this fluid release as the temperature rises, thereby causing fan speed increase for increased cooling air flow through the radiator. As temperature falls, conversely, fluid release or escape is increased to cause fan speed to decrease. One difficulty experienced with this type of control occurs when maximum cooling is required. Under such condition oil flow from the fan drive housing is prevented for maximum fan speed. But, since the engine speed is very high, very high pressures are created on the oil in the fan drive housing. Under this combination of conditions, the high oil pressure in the housing can damage the fan drive components. A pressure relief valve has been provided in the past to release this pressure, but the resulting system with multiple oil lines is very complex and cumbersome.